Non-small cell lung cancer (NSCLC) is the most common cancer killer in the world and is usually diagnosed at an advanced stage that precludes curative surgery. In contrast to the progress targeting driver mutations in NSCLC, there are currently no treatments targeting tumor suppressor loss. Smad4 is a tumor suppressor that transduces transforming growth factor beta signaling. Reduced Smad4 expression is common in NSCLC and Smad4 deletion initiates and promotes lung tumor formation. Smad4-deficient tumors have increased DNA damage while Smad4-deficient cells have reduced DNA repair and hypersensitivity to DNA topoisomerase inhibitors and some DNA repair inhibitors. We hypothesize that the defective DNA repair phenotype of Smad4-deficient NSCLC can be therapeutically exploited if the DNA repair genes critical for survival of Smad4-deficient lung cancer cells are identified and targeted. To test this hypothesis, we propose two aims (1) To determine the DNA repair genes and pathways critical for growth and survival in Smad4-deficient lung cancer cells at baseline and with chemotherapeutic treatment. We will use a DNA repair pathway focused synthetic lethal screen to identify functional dependencies in Smad4-deficient cells at baseline and after treatment with compounds to which Smad4-deficient cells are hypersensitive. (2) To genetically and pharmacologically validate putative synthetic lethal genes in Smad4-deficient cells and tumors. Putative synthetic lethal targets identified in Aim 1 will be genetically validated in vitro and then in vivo to verify that screen detected targets are synthetic lethal with Smad4 down regulation and/or specific drug treatment. Genetically validated targets will be pharmacologically validated in vitro and in vivo as a critical step in transitioning potential targets to clinically applicable treatment approaches. These studies will lay the ground work for clinical trials using reduced Smad4 expression as a biomarker for the selection of treatment regimens designed to exploit the biologic characteristics of these tumors.